The invention relates to a device for ophthalmologically treating the eye using at least one treatment laser beam to ablate parts of the cornea and a fixation light beam that is provided to be fixated by the patient.
In photorefractive keratectomy (PRK), an ametropia of the human eye is corrected by partly reshaping the cornea. A special PRK method that is appreciably (gaining importance at the present time is LASIK. In the LASIK method, a lid (“flap”) is cut in the cornea and folded back. Then a UV laser beam (normally an excimer laser beam having a wavelength of 193 nm) is directed at the exposed parts (laid bare by the lid) of the cornea in order to remove (to ablate) material at that point. After the desired ablation, the lid is shut again and consolidates with the cornea.
The present invention relates generally to PRK and, particular, the LASIK method.
In the photorefractive ophthalmological method, it is important to position the eye precisely with respect to the laser radiation, in particular the ablation beam, used, i.e. in the case of every laser pulse that impinges on the eye in an ablating manner, the system must “know” precisely the point at which the laser beam impinges on the eye. For this purpose, so-called “eye trackers” are used in the prior art. These are devices with which the respective instantaneous position of the eye can be determined in order to control the laser beam in accordance with said determined position. In this connection, the laser beam is guided, for example by means of a scanner, temporally and spatially over the eye surface to he treated (in the case of LASIK, for example, in the stroma), the temporal and spatial control of the laser spot (focus spot) being such that a desired ablation profile is removed (ablated). In this connection, a so-called fixation light beam has to be used in the prior art. A fixation light source is positioned in such a way that the patient can fixate it visually. The patient is asked to do this. This has the object of arresting the eye as constantly as possible in that the patient fixates uninterruptedly the fixation light source. Since the patient has to recognize the fixation light source in this process, it goes without saying that the fixation light source emits a fixation light beam having wavelengths in the visible range, for example, in the green range.
However, the patient does not generally succeed in fixating the fixation light beam in a completely uninterrupted manner. In the prior art, therefore, the said “eye trackers” are known, i.e. optoelectronic systems, with which movements of the eye can be detected. Such movements occur if the patient (involuntarily) “loses sight”, literally, of the fixation light source for a more or less short period of time. If the patient fixates the fixation light source in the ideal way, the fixation light is imaged precisely on the fovea. If, on the other hand, the patient loses sight of the fixation light source, the fixation light is no longer imaged on the fovea, but on another point of the retina, that is to say next to the fovea, to a greater or lesser extent remote from it. The said eye tracker of the prior art, which is assumed herein to be known, detects, for example, movements of the eye by recording the pupil by means of a camera and an image evaluation in which movements of the pupil are detected. The treatment laser beam is then controlled in such a way that such detected eye movements are taken into account and the ablation takes place precisely in accordance with the desired ablation profile despite the eye movements.